Cannondale Frameset, Which Headset

I bought a used Cannondale mountain bike and wanted to replace the headset. The bike shop called me today to tell me they had nothing that would fit and couldn't make any suggestion as to what kind of headset would fit.

Basically the bottom race (on the fork) is a different size than what they stock.

Seems to me that if anyone should know, they would... THEY'RE supposed to be the experts, right?

The frame is a Cannondale with a Rock Shocks suspension fork. The frame is probably a late '80s or early '90s frame... back about the time that Cannondale was selling their "3.0" road frames.

Any help is appreciated... while I'm waiting for a reply here I'll try to locate a bike shop that is a bit more knowledgeable. Seems to me any shop worth its salt would know what headset would fit a particular frame...